The present invention relates to a non-instantaneous disconnection switching system and, more particularly, to a transmission path switching method in a digital transmission system, for switching two path NNI (network node interface) frame signals without instantaneous disconnection, which are applied from opposite apparatuses along different paths having different routing lengths between one another under an external control.
In the conventional non-instantaneous disconnection switching system, when changing routings or transmission path constitution while one of two transmission paths having different routing lengths is used for transmission, the two paths are switched intentionally by non-instantaneous disconnection switching from one to the other under external control.
Transmission between opposite apparatuses which are connected to each other by paths 100 and 101 providing the routing length difference as shown in FIG. 2, is made as follows. To the signal received in an IF (interface) receiver unit 1, a path ID inserter 1a inserts a unique path ID representative of a path which is connected thereto. The signal with the unique path ID given by the path ID inserter 1a is branched by a cross connection switch 2 into two branches, which are transmitted via IF (interface) transmitter units 9 and 10 and paths 100 and 101 to different apparatuses.
In the opposite apparatus, path ID detectors 5a and 6a in the IF receiver units 5 and 6 detect path IDs respectively from the signals applied through the paths 100 and 101 and produce the detected path IDs to a comparator 50. This comparator 50 compares the detected path IDs. If the compared path IDs are identical, a cross connection switch 7 controls the path connection such that the respective outputs of the IF receiver units 5 and 6 are connected to the IF transmitter unit 11. The IF receiver units 5 and 6, like the IF receiver unit 1, are provided with path ID inserters 5b and 6b, but in this case the path ID inserters 5b and 6b are not used.
The IF transmitter unit 11 delays the signal through the routing length of the shorter path for adjusting the routing length difference between the paths 100 and 101. For example, in case where the path 100 is shorter in the routing length than the path 101, the IF transmitter unit 11 provides delay to the signal from the path 100 for the delay adjustment such that an amount of the delay of the signal through the path 100 is the same as that of the signal through the path 101.
After the delay adjustment, a data comparator 11a in the IF transmitter unit 11 compares the signals from the respective paths 100 and 101 for a check as to whether the two signals are coincident. In response to the detection of the coincidence; and of the signals, a selector 11b switches the path 100 over to the path 101. In this way, the non-instantaneous disconnection switching of the paths may be effected.
When the non-instantaneous disconnection path switching is effected by the selector 11b, a path ID detector 11c detects path IDs from the signal through the path 100 before the switching and the signal through the path 101 after the switching.
The IF transmitter units 9 and 10, like the IF transmitter unit 11, comprise data comparators 9a and 10a, selectors 9b and 10b, and path ID detectors 9c and 10c. These circuits operate such that signals from the cross connection switch 2 are directly coupled to the respective paths 100 and 101.
According to the above conventional non-instantaneous disconnection switching method, in response to the switching of one of the two paths to the other or vice versa, this switching is confirmed by reading a signal of the switching control terminal of the selector. This means that if the selector is in trouble, resulting in failure of normal switching control so that no signal switching operation is effected, it is impossible to detect that no signal switching operation has been effected. In this case, if the cross connection setting of the signal on the switching side (cross connection switch 7) is released under the assumption that the switching of the signal has been effected, it leads to a disconnection of the high reliability signal that is transmitted by using the non-instantaneous disconnection switching apparatus.